How the concrete calculator works
First the calculator finds the volume. For slabs and footings that's length × width × depth; for round holes it's π × radius² × depth × number of holes. It then adds your wastage and converts the volume to dry materials. Mixed concrete needs about 2,050kg of cement and ballast per m³, split according to the mix ratio:
Concrete mixes for common jobs
| Job | Mix | Typical depth | Cement per m³ |
|---|---|---|---|
| Strip foundations / footings | 1:6 | 225mm+ | ~12 bags |
| Shed bases, patios, paths | 1:5 | 100mm | ~14 bags |
| Driveways, garage floors | 1:4 | 150mm | ~17 bags |
| Fence posts | 1:5 or Postcrete | 1/4 of post length | ~2 × 20kg Postcrete per post |
Tips for a good concrete slab
- Dig out to firm ground, lay and compact 75–100mm of MOT Type 1, then a polythene membrane under internal slabs.
- Build timber formwork and set a slight fall (about 1:80) so water drains off outdoor slabs.
- Don't add extra water to make concrete easier to work: it seriously weakens it.
- Tamp the concrete level with a board, then cover it with polythene and keep it damp for 3–7 days while it cures. Protect it from frost.
- Large slabs need expansion joints about every 4m, or as advised.
Worked examples
A 3m × 2.5m shed base, 100mm thick: 0.75m³, plus 5% = 0.79m³. In a 1:5 mix that's about 1.6 tonnes of dry materials: 11 bags of cement and 1.35 tonnes of all-in ballast (2 bulk bags), with roughly 135 litres of water added gradually. That's a solid day's work with a small mixer and a helper, but cheaper than a minimum ready-mix load. The shed base planner adds the dig, skip and sub-base.
Four fence post holes, 300mm across and 600mm deep: about 0.18m³ with waste, which is 3 bags of cement and around 300kg of ballast (13 small bags). For standard posts, eight bags of Postcrete is often simpler.
Mix it yourself or order it in?
- Under about 1m³: mixing your own with a hired or bought mixer is usually cheapest. Each mixer load is roughly one bag of cement with its ballast.
- 1–3m³: consider a mix-on-site (volumetric) lorry. It mixes exactly what you need at your gate, and you pay only for what's used.
- Over 3m³: standard ready-mix is usually best. Make sure the lorry can get close, or book a pump.
Pouring, curing and strength
Pour in one continuous session, tamp out air pockets, and level with a straight timber board in a sawing motion. Concrete hardens over days, not hours: you can usually walk on it after a day or two, but it keeps gaining strength for about four weeks. Keep it damp and covered for the first several days, and don't pour if frost is forecast. Freezing water in fresh concrete ruins the surface. For driveways and garage bases, a steel reinforcing mesh set in the middle of the slab helps control cracking.
Common concrete mistakes
- Too much water. Sloppy concrete is easier to spread but much weaker and more likely to crack. Add water little by little.
- No sub-base. A slab poured straight onto soil can settle and crack. Lay compacted MOT Type 1 first: see the MOT Type 1 calculator.
- Ignoring the depth. A 10mm-deeper slab is 10% more concrete. Measure the formwork depth at several points before ordering.
For trenches and footings, the foundation calculator also covers the dig and trench blocks. The excavation calculator works out the spoil. More in concrete, sub-base and paving.